Milo g



(No Modem M. G. KELLOGG.

MULTIPLE SWITGHBOARD.

Patented Oct. 26,1897.

his/am 12m UNITED STATES PATENT rrtca,

MILO G. KELLOGG, OF CHICAGO, ILLINOIS, ASSIGNOR TO TIIE KELIJOGGSlVITCIIlOARD AND SUPPLY COMPANY, OF SAME PLACE.

M ULTIPLE swlTcHBoARD.

SPEGIFICATION forming' part of Letters Patent NO. 592,423, dated October26, 189' 7.

Application filed February 25, 1895. Serial No. 539,620. (No model.)

- To all witam z't mrty/ coltccrn;

Be it known that I, MILO G. KELLOGG, of

Chicago, in the county of Cook and State of Illinois, have inventedcertain new and useful Improvements in Multiple Switchboards forTelephone-Exchanges, of which the following is a full, clear, concise,and exact description, reference being had to the accompanying drawings,formingapart of this specification.

My invention relates especially to a metallic-circuit telephone-exchangesystem.

It consists in a system of calling, switching, and clearing-out signalswhich I shall dcscrbe and claim in detail and in which the two sides ofthe lines are permanently bridged or cross-connected by their respectiveline-annunciators and one side of the circuit of two lines connected forconversation is left grounded through a clearing-out annunciator.

In the accompanying drawings, Fignre 1 illustrates the centraI-officeapparatus, and Fig. 2 the subscriberis-station apparatus, of myinvention.

In Fig. 1, A and A' are two multiple switchboards, to each of which allthe lines of the exchange are connected.

As many boards o r sections may be used as are necessary for theexchange in which the system is used.

Each switchboard or section has a switch for each line of the exchange.These switches are marked 88. Each switch has two contact-pieces,(marked g j,) with which, respectivel y, the two contact-pieces of aloop-switch pln g are connected when a plug is placed in to the switchfor switching. These loop switch plugs are inarked D D in the drawings.Each switch has also two other contact-pieces, (marked a b;) which areinsulated from the other contact-pieces and are normally out of contactwith each other, but when a switchplug is inserted into a switch itpresses the contact-piece a. into contact with b. lVhen the plug iswithdrawn, the contact between a and b is automatically broken.

One of the last-mentioned contact-pieces, as b, is at or extends to ornear the front of the switch, so that an operatoris test contact-piccemay be readily connected to it for testing.

Z Z are the switch-holes adapted to receive the plugs, and c c are therubber pieces on which the metal parts are mounted.

The two contacts g j of each switch are connected to the main-linecircuit, as will hereinafter be described, and may therefore be calledthe main-line contacts of the switch. The two contacts a h of eachswitch are connected in a local system for testing, as will hereinafterbe described, and may therefore be called the local 7' contacts of theswitch.

The loop-switch plugs D D are of the shape substantially as shown, andeach plug has two contacts, (marked m m'.) Vhen a plug is placed into aswitch, its contact m forms connection with the contact g of the switch,and its contact m' forms connection with the contact j of the switch,while the rubber insulation of the plug presses the piece a of theswitch into contact with the piece b. The two latter pieces a. b are,however, always insulated from the .pieces g j.

The switches and plugs may be of the shape and constructionsubstantially as shown. They may, however, be greatly changed in shapeand construction and yet have substantially the same switch parts,Operating in the same manner, as heretofore described.

'w w are the line,-annunciators, of which there is one for each line.These annunciators are distributed among the several boards or Sectionsof the exchange and each is placed at the section where its subscriberiscalls are to beanswei'ed.

Each operator has as many pairs of loopswitch plugs D D as she may needin order to properly attend to the lines which are allotted to her. i

Y Yare switches, one foreach pair of plugs. Each switch Y has twoswitch-levers and two pairs of contact-bolts,on which, respectively, thetwo levers may be alternately placed. It has also another or lifthcontact-bolt,on which one of the leVers maybe placed. The switchleversare adapted to occupy three positions at the will of the operator, inone of 'which they are in contact with one pair of said bolts, inanother of which they are in contact with the other pair of said bolts,and in the third position of which one of them is in contact with saidfifth bolt.

IOO

IOl

t t are the operators telephones, of which there is one for eachoperator.

B B are calling-generators, of which there may be one for each operator,or one genera- 'tor may answer for several or all of thevoperators.

'u lv are clearing-out-annunciators, of which there is one for each pairof plugs. v

B' is a test-battery connected into the local test-Circuit, as willhereinafter be described.

The two contact-pieces m 'm of a pair of plugs are connected together bya flexible conductor, and the two contact-pieces 'm' m' are connectedtogether by another flexible conductor. The two fiexible conductors of apair of plugs are connected to the two levers of the switch Yof the pairof plugs. The flexible conductors are long enough so that a switch-plugmay be Vplaced in any switch of the section at which it is located. Thebolts of one pair of contact-bolts of each switch Y are connectedtogether through the operatoris telephone. The bolts of the other pairare connected together by a circuit connection, from the center' ofwhich is a connection to one side of the calling-generator B. .The otherside of the generator is connected with ground. The last or fifthcontact-bolt of each switch Y is connected to one side of theclearing-out annunciator 'v of the pair of plugs and the other side'ofthe annunciator o is connected to ground.

One side or branch of each metallic-circuit line is connected to one ofthe main-line contacts, say g, of each switch of the line, and the otherside or branch is connected to the other main-line contact, say j, ofeach switch of the line.

The two sides or branches of each line are bridged or connectedvtogether at the central oflice through the line-annunciator w of theline.

All the contacts a a of all the switches of the exchange are connectedtogether by a circuit connection. One side of the battery B' isconnected to this circuit connection.

All the contacts b b of the switches of a line are connected together.

The operatoifs telephone has two coils. From the Circuit-Wire whichconnects these coils a connection is run to the other side of saidbattery B'.

The circuit connections, both main-line and local, are shownsubstantially as above described, For convenience of illustration themain-line circuit and connections are represented by solid lines and.the local circuits and connections are represented by broken lines.

In the ubscribers-station apparatus shown in Fig. 2, 1 is thetelephone-switch; 2, the subscriber7s telephone; 8, the subscriber7scallinggenerator; 4, his signal-receiving bell, and 5 a resistance-coilof the resistance which will hereinafter be described. V

The calling-generator has the usual automatic device by which itsarmature is automatically short-circuited while it is not beingoperated. It should preferably be an alternate-current generator. I

The switch 1 has alever and three insulated contact-pieces, with two ofwhich the lever is in contact when the telephone is on the switch, andwith the third of which the lever is in contact when the telephone isoff the switch.

One of the first-mentioned contacts is connected with-the ground.

Both sides of the line are connected to the switch-lever. In one sideare the calling-gen-` erator. and the signal-receiving bell and in theother side' are the subscriberis telephone and the resistance-coil 5.The circuits to the switch-contacts are substantially 'as shown' andsuch that when the telephone is on the switch the telephone isshort-circuited, and when the telephone is off from the switch thecallin g-generator and si gnal-receivin g bell are short-circuited.

The resistance of one side of the line from I the switch-lever to theconnection with one side of theline-annunciator should normally, or whenthe subscribeifs telephone is not oif from the switch, be substantiallythe same as the resistance of the other side of the line from theswitch-lever to the other side of the line-annunciator. This may beobtained by making the resistance of the coil, 5 of the right amounttoproduce this equality of resistance. Of course the lines themselvesmay be constructed so as to obtain this equality of resistance.v

All the annunciators should be constructed so as to offer highretardation to alternating currents, and the line-annunciators should,for the purpose hereinafter indicated, be of much higher resistance thanthe clearing-out annunciators.

The operation of the system is as follows:

i When a subscriber operates his calling-generator, he |sends a signalover his metallic Circuit which operates his line-annunciator. Theoperator who receives the indication then places one of her switch-plugsD into the switch of the line and places the levers of the switch Y t`owhich the plug belongs in the position in which the two sides of hertelephone are connected to the two levers of the switch. Her telephoneis then in complete or closed metallic circuit with the line, and shewill receive verbally the order of the subscriber. This complete circuitexists because the two contacts of the plugs (which are connected to thetwo levers -of the switch Y) are by their contact with the two contacts'grounded through the generator B.) Our- IOO llO

rent from the operators generator will then pass from ground at thecentral office through the two sides or branches of the metallic circuitof the line wanted in split 01' derived circuit to the ground at thesubscribens station. As -the two branches of the line are of the sameresistance, as much current will pass through one branch as the other.The current which passes through the signal-receiving bell of thesubscriber wanted will be sufficient to ring his bell. The operator thenmoves the swtch-levers of the switch Y so that one of the levers is onsaid fifth contactbolt, connected to ground through the clearing-outannunciator of the pair of plugs. The two lines are thus connectedtogether into a complete metallic circuit for conversation, and one sideof the circuit is grounded through the clearing-out annunciator. Vhenthe lines are thus connected, their combined circuit is bridged orcross-connected through the annunciators of both lines. As theseannunciators offer high retardation to telephoneand other alternatingcurrents, substantially, none of the telephone-current generated' duringthe conversation will pass through them.

When the call is sent by the operator, the subscribeifs line-annunciatorwill not be operated for the following reason: The resistances from thegenerator to each side of the line-annunciator are the same, as are also'the resistances from each side of the annunciator through the two sidesor branches of the line to the subscriber's ground. None of thecalling-current will therefore pass through the line-annunciator. Theseresistances should be equal with sufficient approximation so that enoughcurrent will not pass through the line-annunciator to operate it.

The clearing-out system is as follows: Each subscribefis apparatus isconnectedl into his circuit so that his calling-generator is in thecircuit between his ground and the ground connection through aclearing-out annunciator when the line is switched for conversation. Theclearing-out signal is sent after the subscriber has placed histelephone on the switch. The current generated will therefore divideinto four parts, passing through four branch or derived circuits. One ofthese circuits is from the subscribefis ground through his generator'and one branch of the line to the clearing out annunciator, and thenceto ground. Two other circuits through which part of the current passesare through his metallic-circuit line to the central office, and thencein split-circuit through the two line-annunciators. The fourth circuitis by way of the subscribeifs station of the other line. If, when theclearing-,out signal is sent, the telephone of the other subscriber isnot yet placed on its switch, the circuit is through themetallic-circuit line of the other subscriber. If, however, bothsubscribers have their telephones on their switches, it is from theground at one station through one side will-operate the clearing-outannunciators,

and in the latter case there will not be the necessity of as great adifference of resistance between the line-annunciators and theclearing-out annunciators as wouldiotherwise be necessary to make thesystem operative.

The subscribers calling-generators should be of such power and adjustment to the other apparatus and circuits and the other apparatus andcircuits should be so adjusted to each other as to resistance andscnsitiveness of operation and to the calling-generator that when a callis sent from any station and the line is not switched for conversationthe lineannunciator will invariably be operated; but when two lines areconnected together, as above described, and their circuit is groundedthrough a clearing-out annunciator the operation of thecalling-generator of either subscriber will invariably opera-te theclearin gout annunciator and invariably fail to opererate eitherline-annunciator.

The test system is as follows: Each operator can place any contact-piecem of any of her switch-plugs into contact for testing with anycontact-piece Z) of any switch located at her board. Then she has thusplaced a plug into contact with the piece Z) of any line (her telephonebeing then in connection with the plug) and the line is switched at anyboard for conversation, a complete circuit is established which containsthe battery B' and one coil of her telephone and the telephone willsound or click, indicating that the line is switched for use. Thiscomplete circuit may be traced as' follows: From the switch-plug used intesting through one coil of the operators telephone, thence through thebattery B' to the circuit connection which connects together all thecontacts a a of all the switches of the exchange, thence to the contacta of the switch at which the line tested is switched, thence to thecontactb of that switch, and thence to the contact Z) to which the plugis applied for testing. If the line is not switche d at any board, nosuch complete circuit is established on testing, and the operator Willget no click or sound in her telephone. She can therefore determine bytesting whether or not the line is in use.

It is evident that the subscribefis signalreceiving bell might also beplaced in the ground connection at his station. In that case all thecalling-current which the operator sends over his line would passthrough his bell. For the purpose heretofore indi- IOO IIO

cated the resistances of the two branches of the line should in thiscase also be made substantially alike. Certain features of my inventionare 'also applicable to systems in which the line-annunciators areprevented from being ,operatedv when either the operator calls or aclearing-out signal is sent by other instrumentalities than'thoseheretofore indicated. My invention in these respects is broad enough toinclude these variations.

The system herein claimed is limited to a system in which the two sidesof the lines are permanently bridged or cross-connected by theirrespective line-annunciators and one side of the circuit of two linesconnected for conversation is left grounded through a clearing-outannunciator, as I have other applications pending on other systems in'which broader claims are pending, and it is not my intention inprosecuting this application to waive or abandon anything claimed insuch other applications.

I claiIn- 1. In a telephone-exchange system, metallic-circuit lines eachnormally or while not switched for use ungrounded at the central oflice,multiple switchboards, switches, one switch for each line on each'board,each switch having two contact pieces or terminals to whichrespectively the two sides or branches of its lines are connected, andmeans at each board to connect together any two lines into a metalliccircuit for oonversation by connecting together their respective contactpieces or terminals at that board, in combination withline-annunciators, one of each line located at a switchboard andpermanently bridged across between the two sides or branches of itsline, a clearing-out annunciator grounded on one side and connected 011its other side to one side of a metallic circuit established byconnecting any two lines together for conversation; a calling-generatorat each subscribers station located in the closed metallic circuit withboth sides of its line whenever the generator is operated to send anysignal (either primary or clearing-out) a'nd means for grounding eachclosed metallic circuit at the subscriber7s station while the subscribersends a clearing-out signal with the ground connection on such side ofthe generator that the generator and the clearing-out annunciator are ina closed' ground-circuit which comprises that side or branch of the lineto which the clearing-out annunciator is directly connected.

2. In a telephone-exchange system, metallie-circuit lines, each normallyor while not switched for use ungrounded at thevcentral office, multipleswitchboards, switches, one switch for each line, on each board, eachswitch having two contact pieces or terminals to which respectively thetwo sides or branches of its line are connected, and means at each boardto connect together any two lines into a metallic circuit forconversation by connecting together their respective contact pieces orterminals at that board, in combination with line-annunciators, one foreach line, located at a switchboard and permanently bridged acrossbetween the two sides or branches'of its line, a clearing-outannunciator grounded on one side and connected on its other side to oneside of a metallic circuit established by connecting any two linestogether for conversation said lin e-annunciator being of a resistancerelatively high to the clearing-out annunciator, a calling-generator ateach subscribefis station located in the closed metallic circuit withboth sides of its line whenever the generator is operated to send anysignal (either primary' or clearingout) and means for grounding eachclosed metallic circuit at the subscribeifs station while the subscribersends a clearing-out signal, with the ground connection on such side wof the generator that the generator and the clearing-out annunciator arein a closed ground-circuit which comprises that side or branch of theline to which the clearing-out p annunciator is directly connected.

3. In a telephone-exchange system, metallie-circuit lines, each normallyor while not switched for use ungrounded at the central office, multipleswitchboards, switches, one switch for each line on each board, eachswitch having two contact pieces or terminalsto which respectivel y thetwo sides or branches of its line are connected, and means at each boardto connect together any two linesinto a metallic circuit forconversation by connecting together their respective contact pieces orterminals at that board, in combination with line-annunciators one foreach 'line located at a switchboard and permanently bridged acrossbetween the two sides or branches of its line, a clearng-out annunciatorgrounded on one side and connected on its other side to one side of ametallic circuit established by connecting any two lines together forconversation, said line-annunciator being of a resistance relativelyhigh to the clearing-out annunciator, a calling-generator at eachsubscribefis station located in the closed metallic circuit with bothsides of its line whenever the generator is operated vto send -anysignal (either primary or clearingout) and means for grounding eachclosed metallic circuit at the subscriber7s station, while thesubscriber sends a clearing-ont signal with the ground connection onsuch side of the generator that the generator and the clearing-outannunciator are in a closed ground-circuit which, comprises that side orbranch of the line to which the clearing-out annunciator is directlyconnected, said annunciators and generators 'being of such constructionand such relation to each other and to the circuits that when agenerator is operated while its line is not switched itsline-annunciator is operated and when a generator is operated, while itsline is switched the line-annunciators are not operated, but theelearing-out annunciator through which the circuit of the two unitedlines is grounded is operated.

4. In a telephone-exchange system, metallic-circuit lines, each normallyor while not switched for use ungrounded at the central office, multipleswitchboards, switches', one switch for each line on each board, eachswitch having two contact pieces or terminals to which respectively thetwo sides or branches of its line are connected, and means at each boardto connect together any two lines into a metallic circuit forconversation by connecting together their respective contact pieces orterminals at that board in combination with line-annunciators, one foreach line located at a switchboard and permanently bridged acrossbetween the two sides or branches of its line, a clearing-outannunciator grounded on' one side and connected on its other side to oneside of a metallic circuit established by connecting any two linestogether foi` eonversation; a calling-generator at each subscriber'sstation located in the closed metallic circuit with both sides of itsline whenever the generator is operated to send any signal (eitherprimary or clearingout) and means for grounding each closed metalliccircuit at the subscriberls station, while the subscrber sends aclearing-out signal, with the ground connection on such side of thegenerator, that the generator and the clearing-out annunciator are in aclosed ground-circuit which comprises that side or branch of the line towhich the clearing-out annunciator is directlyconnected; saidannunciators and generators being of such const-ruction that when agenerator is operated while its line is not switched, theline-annunciator is operated, and when a generator is operated while itsline is switched the lineannunciators are not operated, but theclearing-out annunciator through which the circuit of the two unitedlines is grounded is operated. i

5. In a telephone-exchange system, metallie-circuit lines, each normallyor while not switched for use ungrounded at the central ofiice, aswitchboard, switches, one switch for each line on said board, eachswitch having two contact pieces or terminals to which respectively thetwo sides or branches vof its line are connected, and means to connecttogether any two lines into a metallic circuit for conversation byconnecting together their respective contact pieces or terminals, incombination with line-annunciators one for each line permanently bridgedacross between the two sides or branches of its line, a clearing-outannunciator grounded on one side and connected on its other side to oneside of a metallic circuit established by connecting any two linestogether for conversation, a calling-generator at each subscribefisstation located in the closed metallic circuit with the two sides of itsline whenever the generator is operated to send any signal (whetherprimary or clearing-out) and means at each subscriberis station forgrounding his closed Inei tallic circuit while he sends a clearing-outsignal, with the ground connection on such side of the generator thatthe generator and the clearing-out annunciator are in a closedground-circuit which comprises that side or branch of the line to whichthe clearing-out annunciator is directly connected.

6. In a telephone-exchange system, metallie-circuit lines, each normally01' while not switched for use ungrounded at the central ofiice, aswitchboard, switches, one switch for each line on said board, eachswitch having two contact pieces or terminals to which respectively thetwo sides or branches of its line are connected, and means to connecttogether any two lines into a metallic circuit for conversation byconnecting together their respective contact pieces or terminale., incombination with line-annunciators, one for each line permanentlybridged across between the two sides or branches of its line, aclearing-out annunciator grounded on one side and connected on its otherside to one side of a metallic circuit established by connecting any twolines together for conversation, said line-annunciators being of aresistance relatively high to the clearing-out annunciators; acalling-generator at each subscriberls station located in the closedmetallic circuit with the two sides of its line whenever the generatoris operated to send any signal (whether primary or clearing-out) andmeans at each IOO subscriberis station for grounding his closed metalliccircuit while he sends a clearing-out signal, with the ground connectionon such side of the generator, that the generator and the clearing-outannunciator are in a closed ground-circuit which comprises that side orbranch of the line to which the clearing-out annunciator is directlyconnected, said annunciators and generators being of such constructionand such relation to each other and to the eircuits, that when agenerator is operated while its line is not switched its lineannunciatoris operated, and when a generator is operated while its line is switchedthe line-annunciators are not operated, but the elearing-out annunciatorthrough which the circuit of the united lines is grounded is operated.

7. In a telephone-exchange system, metallie-circuit lines, each normallyor while not switched for use ungrounded at the central office, aswitchboard, switches, one switch for eac'h line on said board, eachswitch having two contact pieces or terminals to which respectively thetwo sides or branches of its line are connected, and means to connecttogether any two lines into a nietallic circuit for conversation byconnect-ing together their respective contact pieces or terminals, incombination with line-annunciators, one for each line, permanentlybridged across between the two sides or branches of its line, aclearingout annunciator grounded on one side and IIO connected on itsother side to one side of a I metallic circuit established by connectingany two lines together for eonversation, a calling-generator at eachsubscriberls station located in the closed metallic circuit with the twosides of its line Whenever the generator is operated to send any signal(whether primary or clearing-out) and means at each subscribeifs stationfor grounding his closed metallic circuit while he sends a clearing-outsignal with the ground connection on such side of the generator, thatthe generator and the clearing-out annunciator are in a closedground-circuit which comprises that side or branch of the line to whichthe clearing-out annunciator is directly connected; said annunciatorsand generators being of such construction that when a generator isoperated While its line is not switched the line-annunciator is operatedand when a generator is operated While its line is switched, thelineannunciators are not operated but the clearing-out annunciatorthrough which the circuit of the two united lines is grounded isoperated.

8. In a telephone-exchange system, metallic circuit lines, multipleswitchboards, switches, one switch for each line on each board, eachswitch having two contact pieces or -terminals to which respectively thetwo sides or branches of its line are connected, and means at each boardto connect together any two lines into a metallic circuit forconversation by connecting together their respective contact pieces orterminals at that board, in combination with line-annunciators, one ofeach line located at a switchboard and permanently bridged acrossbetween the two sides or branches of its line, a clearing-outannunciator grounded on one side and connected on its other side to oneside of a metallic circuit established by connecting any two linestogether for eonversation, a calling-generator at each subscribefsstation in the closed metallic circuit of its -line wheni ever it isoperated to. send any signal (either primary or clearing-out) and meansfor groundin g each closed metallie circuit at the subscriberis stationwhile the subscriber sends a elearing-out signal, withthe ground jconnection on such side of the generator that the generator and theclearing-out annuncii ator are in a closed ground-circuit which com- 1 lprises that side or branch of the line to which the clearing-outannunciator is directly connected.

9. In a telephone-exchange system, met-allic-eircuit lines, multipleswitchboards, switches, one switch for each line on each board, eachswitch having two contact pieces or terminals to which `respectively thetwo sides or branches of its line are connected,

and means at each board to connect together any two lines into ametallic circuit for con- Versation by connecting together theirrespective contact pieces or terminals at that board,in combination withline-annunciators,

one for each line located at a switchboard and permanently bridgedacross between the two sides or branches of its line, a clearingoutannunciator grounded on one side and connected on its other side to oneside of a m eta'llic circuit established by connecting any two linestogether for eonversation, a callinggenerator at each subscribefisstation in the closed metallic circuit of its line whenever it isoperated to send any signal (either primary or clearing-out) and meansfor grounding each closed metallic circuit at the subscribers stationwhile the subscriber sends a clearing-out signal, with the groundconnection on such side of the generator that the generator and theclearing-out annunciator are in a closed ground-circuit which comprisesthat side or branch of the line to which the clearing-out annunciator isdirectly connected; said annunciators and generators being of suchconstruction that when a generator is operated while its line is notswitched the line-annunciator is operated and When a generator isoperated while its line is switched the line-annunciators are notoperated but the clearing-out annunciator through which the circuit ofthe two united linesis grounded is operated.

1 O. In a telephone-exchange system, metallic-circuit lines, aswitchboard, switches,

w one switch for each line on said board, each switch having two contactpieces or terminals to which respectively the two sides or branches ofits line are connected and means to connect together any two lines'intoa metallic circuit for eonversation by connect-ing together theirrespective contact pieces or terminals, in combination withline-annunciators one for each line permanently bridged across betweenthe two sides or branches of its line, a clearing-out annunciatorgrounded on one side and connected on its other side to one side of ametallic circuit established by connecting any two lines together forconversation, a calling-generator at each subscriberis station in theclosed metallie circuit of its line wh enever it is operated to send anysignal (whether primary or clearing-out) and means at each subscriber7sstation for grounding his closed metallic circuit while he sends aclearing-out signal, With the ground connection on such side of thegenerator that the generator and the clearing-out annunciator are in aclosed ground-circuit which comprises that side or branch of the line towhich theclearing-out annunciator is directly connected.

11. In a telephone-exchange system, metallie-circuit lines, aswitchboard, switches, one switch for each line on said board, eachswitch having two contact pieces or terminals to which respectively thetwo sides or branches of its line are connected and means to connecttogether any two lines into a metallic circuit for eonversation byconnecting together their respective contact pieces or terminals, incombination with line-annunciators one for each IOO IIO

line perinanently hridged across between the two sides or branches ofits line, a clearingout annnnciator grounded on one side and connectedon its other side to one side of a nietallic circuit established byconnectin g any two lines together for conversation, a callinggeneratorat each subscribeifs station in the closed metallic circuit of its linewhenever it is operated to send any signal (whether priniary orclearing-ont) and means at each subscribers station for grounding hisclosed Inetallic circnit while he sends a clearing-out signal, with theground connection on such side of the generator that the generator andthe clearngout annnnciator are in a closed ground-circuit whichcomprises that side or branch of the line to which the clearing-outannunciator is directly connected, said annnnciators and generatorsbeing` of such construction that when a generator is operated while itsline is not switched the line-annunciator is operated and when agenerator is opcrated while its line is switched the line-annunciatorsare not operated but the clearingout annunciator through which thecircuit of the two united lines is grounded is operated.

12. In a telephone-exchange system, two met-allic-circuit lines and twoline-annunciators, one for each line, through which the line ispermanently closed at the central office, said lines being telnporarilyswitched together for conversation into a single metallic circuit, incombination with a clearing-out annunciatorin a circuit connection fromsaid inetallic circuit to ground, and a calling-generator at eachsubscriberls station in circuit with said line-annunciators andclearing-out annunciator when operated to send a clearing-out signal,substantially as set forth.

In testiinony whereof I have hereunto subscribed iny name.

MILO G. KELLOGG.

lfitnesses:

A. M. MoLAcHLEN, LLoYD B. WIGHT.

